It has been known that 5-hydroxy-1-methylhydantoin is useful as plant growth regulator (Japanese Patent Laid-Open No. Sho-57-114578), hypoglycemic agent, diuretic (Japanese Patent Laid-Open No. Sho-60-188373), hypolipemic agent (Japanese Patent Laid-Open No. Sho-6245525), improving agent for renal function (Japanese Patent Laid-Open No. Hei-03-72463), eliminating agent for active oxygen and free radical (Japanese Patent Laid-Open No. Hei-09-227377), therapeutic agent for intractable vasculitis (Japanese Patent Laid-Open No. 2000-212083), agent for hypoaqlbuminaemia (Japanese Patent Laid-Open No. 2002-241283), etc. and a process for producing the compound is disclosed in Japanese Patents Laid-Open Nos. Sho-57-114578, Sho-60-188373 and Sho-61-122275. In the process for the production disclosed in the above-mentioned gazettes, 5-hydroxy-1-methylhydantoin is recrystallized from ethyl acetate.
When the samples which were recrystallized from organic solvents such as ethyl acetate, acetonitrile, acetone, a mixture of ethanol and hexane, a mixture of tetrahydrofuran and chloroform, etc. were analyzed by means of infrared spectrophotometry or X-ray powder diffraction method, all cases gave the same crystal form and it has been believed that there is no crystal polymorphic form of 5-hydroxy-1-methylhydantoin. As a result of analyses of the residual solvent in the crystal form obtained by recrystallization from acetone, about 1,300 to 1,700 ppm of acetone was detected as a residual solvent. Under such circumstances, investigations were conducted for a method of removing the residual solvent using drying by heating, drying by means of spray-drying method or the like, but its removal has not been successful.
As to the compounds used as drugs, it is preferred that toxic residual solvent are made as small as possible. Especially in patients suffering from renal failure, waste products and toxic substances are not excreted but accumulated in the body due to the disorder of renal functions and, therefore, toxic residual solvent or the like is to be made as little as possible in drugs such as that for renal failure. However, as mentioned above, considerable amount of solvent for recrystallization remains when 5-hydroxy-1-methylhydantoin is recrystallized from organic solvent such as acetone or ethyl acetate and, therefore, that is not preferred as a material for drugs, particularly the drug for renal failure, which are demanded to be highly safe. Accordingly, there has been a demand for obtaining a pure product which substantially contains no residual solvent.